1. Field of the Invention
The present invention relates to a blower which is of sound-damping structure.
2. Description of the Related Art
FIG. 13 of the accompanying drawings is a vertical side view in section showing a blower which is of sound-damping structure, as disclosed in e.g. Japanese Unexamined Utility Model Publication No. 114000/1986. FIG. 14 is a front view in section of the blower of FIG. 13. In these Figures, reference numeral 1 designates an impeller which functions to raise the pressure of air or other gases and to deliver it. Reference numeral 2 designates an electric motor which is used to drive the impeller 1. Reference numeral 3 designates a fan casing which comprises a hard porous layer prepared in a porous structure by foaming or sintering a plastic material. Reference numeral 4 designates a fan inlet. Reference numeral 5 designates a fan outlet.
The conventional blower, which is constructed as stated above, draws in it air or other gases through the fan inlet 4 under the action of the impeller 1 rotated by the electric motor 2, and causes the air or gases to flow out from the fan outlet 5. In the course of moving the air from the inlet to the outlet, blower noise which is produced by the impeller 1 emits from the fan inlet 4, the fan outlet 5, and the surface of the fan casing 3. Because the fan casing 3 is made of the porous layer as stated above, most part of the blower noise can be absorbed and damped in the porous layer to suppress the noise which is emitted outside from the inlet and the outlet.
However, in the conventional sound-damping structure for a blower, the porous layer which forms the fan casing 3 is equal in specific gravity in the direction of thickness of the layer and in a direction of surface of the layer. As a result, the layer has to be great in thickness in order to improve sound absorption performance. This creates problems in that the size, the weight, the production cost and the like of the blower are increased. If the porosity in the porous layer is increased as a result of having given importance to sound absorption effect, the porous layer will have a high rate porosity equality in its entirety, the air can leak outside through the fan casing 3, creating a problem wherein aerodynamic performance is lowered.